Well, depends what you mean by opening day. First, there will probably be closed previews for media and/or VIP invitees. Then, there will probably be paid previews open to the public, basically special days/nights/times when you can buy a ticket (Pixar Pier preview was $200+) and visit. This could be as early as May, and people are speculating Memorial Day weekend.
Once all that's done and the land officially opens... you might have some options other than rushing to get in line. There might be some paid options, like a tour that includes one or both of the attractions, or a dining package that allows early access to the attractions (like the WDW Toy Story & Fantasyland breakfasts) or comes with a FP to one or both of the attractions.
If you're not looking to do any of those things, then whatever day it opens to the public conventional wisdom would be to get there as early as possible, get a FP for one of the attractions and jump in the standby line of the other attraction (assuming it's not longer than your FP return time for the other one!). MaxPass should be your friend here. Of course, this is what EVERYONE is going to try to do, so whether it will work remains to be seen.
There are other things to consider, like... how will DL rope off the lines and control access to GE? It seems likely that huge lines will immediately form just to grab FPs for each attraction. It might be physically difficult to grab a FP for one and then just "get in line" for the other one in a convenient way that avoids an hours long line. Again, MaxPass might help here. But we don't know how MaxPass will work either. Will you be able to get FPs for both? Or will it be limited to only one of the attractions per day (kind of like Pandora at WDW)? Will you need a FP for an attraction just to get into the land? Will there be single rider lines? Will the attractions break down since they're brand new and still have some bugs to work out? Will the blue milk be any good? All questions that are yet to be answered.
