FOr me, tandem nursing isn't discreet until after I get them all latched on and settled, then I can kinda keep their head in front and my shirt down a bit, but it's not something I would be comfortable with in public b/c I"d have to flash everyone for a few minutes in order to get them on! We went out this weekend and I nursed just one at a time in public, it's much easier! I do not have any trouble getting one baby on and nursing discreetly, but two is just too to hold/manouver, ect.
I know you want to tandem them to keep them on the same schedule, but with your mom and MIL there, you can nurse one, hand that one off to be burped and nurse the second one fairly quickly and keep them sort of close to the same schedule. I do this even now and have no problem getting them back to nursing together if no one is here to help, but with help nursing one and having the helper burp the other works really well for me!