My own experience is that training hard and soft styles at the same time will be counterproductive, since they tend to work in opposite directions. However, my understanding is that our Tai Chi is not just soft, but also hard, and that hard styles also contain soft. The ultimate difference is not so much content as emphasis, which is actually quite important.
We also say that our Tai Chi is not just internal, but also external. It is just that we focus on the internal first and generally get the external along the way as a byproduct. The ultimate endpoint is comparable to what most hard or external styles want, but the journey is very different.
I personally like much of the dialog around ‘tendons’ versus ‘muscles’; however, we must be careful how we use terms. Even things as simple as body parts have different ranges of meaning across languages (e.g., 腰 yao1 vs "waist") or across disciplines (e.g., everyday meaning of blood or 血 xue4 vs. TCM meaning of blood or 血 xue4). Even more importantly, I believe that the Tai Chi classics generally attempt to speak from the deep experience of masters and to call to the burgeoning experience of dedicated students, rather than directly from philosopher to philosopher.
I am far from a master, but recently spent a couple of hours teaching the rudiments of Fajin and had to touch on many of these issues. I repeatedly explained and repeatedly demonstrated. I made numerous citations to the classics as well as my own understanding of the principles from a more modern viewpoint. I made extensive analogies and even made use of inanimate objects to explain what I wanted. One of my touchstones in teaching is that I generally prefer to be clear and wrong than vague and right and so I tried to be excruciatingly specific in terms of what I wanted. If what I say is wrong, a student can very quickly see through what I am saying and make his own judgement. If what I say is vague, a student cannot be sure he fully understands and can remain frustratingly in the dark about what is correct.
After a couple of hours, I think the students began both to understand and to execute the movements somewhat appropriately. I even had a couple perform a medium-force palm strike to my chest, just to feel how much power could be generated by not focusing on the local power in the arms. Even though I wanted the students to feel the impact and so did not want to counter, I had to soften my chest and step back a few paces to avoid coming close to injury. Just to be clear, we generally have very relaxed practices and are closer to considerations of retirement homes than fighting matches.
In trying to explain what I wanted, there were three analogies that seemed to resonate more than others. One was to imagine a spring made of rope (Yin) and a spring made of stone (Yang). Neither has the resilience of a spring made of steel (Taiji). Even a spring made partly of rope and partly of spring (Yin and Yang or 两仪 liang3 yi2, but not Taiji: ) does not work. Nor does steel in the form of a straight wire (No Yin and Yang at all). Envisioning the limbs as generating power with muscles and bones is like focusing on generating power with stone and rope. Focusing on the network of tendons and structure enables the natural spring-like power of Peng energy.
Another analogy I used was to focus on shooting an arrow with a bow. At the moment you fire, you do not try to push the arrow or really add any more energy, you merely release the energy that is already naturally there from drawing the bow. When you Fajin with your arms in a push, you do not actually "push" at the end, you merely release the power your legs have already stored in your arms. If your arms are either like a rope or a stone, no energy can be stored or transmitted. You cannot focus on muscle or bone, but rather must focus on tendons and structure.
This does not mean you do not use any muscle or bone, however. If there is none of yourself in the energy of your arms, your arms will be like ropes and cannot store energy. If there is only yourself in the energy of your arms, your arms will be like stone and cannot transmit energy. If you can figure out how to combine your own energy with the energy naturally present, you can have a resilient energy.
The last analogy I used was that of a whip. I recall Master Yang using this. When you hit something with a whip, you send energy into it with a final flick of the wrist and know from experience where the tip will go; however, you do not directly control the whip, the wave of energy does. When you strike with internal energy, you do not directly control the energy at the target, but rather just know from experience what will happen if you set up the right conditions.
Since I knew all this could sound like a lot of mere talk, I grabbed a Pilates ball and made it leap up from the floor without applying any upward energy myself. I used the nature of the ball. If you strike down into it just right, it will rebound up into the air. This is somewhat how I understand the basics of Fajin: add the right energy and let nature take its course.
If you try to train hard arts at the same time as soft arts, you will focus very much on training your own energy and hardening yourself. This is generally in the exact opposite direction you need to go for Tai Chi, where we want more to be aware of natural energy and to make ourselves as soft as possible. I think it is hard to train how to go from hard to hard and from soft to hard at the same time.