you would have to be in a location to straddle synchronous grid. There may be some places that allow that. Otherwise, the two sources are already in approximately in phase anyways. To put it simply though, you put some devices on one feed and some devices on another feed. So it is a non-issue.

If you have local generation you want to combine with grid power you usually just sync your local generation to the grid. If you don't want to do that you can just use a DC intertie that is local. Basically you'd have two AC -> DC converters and a single DC -> AC converter.