I address the "fairness" issue by choosing partners who are similarly busy, so no one is really getting "short-changed" because they're just as unavailable as I am. So the answer to "how do we grow" is "slowly."
But really, how is it any different than most single people dating these days? The pace of life has grown beyond control for many professionals. People are stretched too thin all across the board. Poly availability is just one more aspect of that.
The only real answer is to slow down and reconsider your priorities. e.g. you "work during the week." Do you work 16 hours every day? Do you need to? If you only work a regular 8 or even 10 hour day, then there are several hours left in the evening for you to use as you choose. If your evenings are filled with dog care, perhaps you could seek partners who are dog lovers and would be willing to accompany you while you take 6 dogs to the park.
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."