2 biologics at the same time are really a last resort. But if I'm understanding correctly, your doctor is not saying he'll be on both - just to finish the remaining doses of Humira before moving on to Stelara? That is unusual. My daughter has been on MANY different biologics and once we get a new biologic approved, we start it - we don't continue with a med that isn't working. Typically it's one dosing cycle - so my daughter was on Humira every 7 days when we switched to Remicade - we just did the 1st Remicade infusion when the next Humira shot would have been due (7 days from her last Humira). It did mean we had several extra Humira shots left in the fridge - I always keep them until they expire.
Sometimes we don't even wait that long - as soon as the new drug is approved, we switch to that (with the doctor's permission). For example, I remember switching to Simponi about 2 weeks from her last Remicade infusion, even though she was getting Remicade infusions every 4 weeks. Her GI was not at all worried.
I'm also kind of surprised your GI is not exhausting Humira as an option before moving on - there are only essentially 4 biologics for Crohn's, so you want to make sure you do everything you can to try to get Humira to work before switching. Normally, you'd increase Humira to weekly and you could also add MTX. And often you'd also try a steroid or EEN, to induce remission while you're waiting for Humira to work.