I can't remember if this was always possible, but I have just tried it now, and it worked. But also brilliantly, I was able to fire again while the med item counts down before taking effect. Surely that hasn't always been the case?
It does not matter whom you heal, party member, yourself, or some other PC. it was always same. You can use med items while in fire delay, and you can fire while you are in "med item delay", no matter if you did apply it to yourself, or someone else did. What you can not do is to use force heal while you are in fire delay, or "med item delay". But you can fire and use items after you've used force heal.