As a church employee, I would say pick one. Here’s the reason:
The parishes are taxed by the Diocese based on their membership numbers.
If you are artificially inflating those numbers it hurts the parish. If you are receiving envelopes from both parishes, you’re hurting the bottom line as well. These are administrative concerns, but they are concerns. (IF you are not giving in both locales)
Do you receive a tax notification from both?
For ex:
If a pastor wants to build a new youth hall, and his records show that there are 400 teens in his parish, he may proceed, asking the Bishop for clearance to go forward. But when push comes to shove, and those kids are not worshipping there, not in formation there, then he has a lot of 'splaining to do.

Many parishes have really messed up databases for this reason. Because people just register in a few places, like it doesn’t matter “just in case” or to cover their bases.
It’s a problem. Likewise with people who move out of state and never tell anyone. We’d like to not keep publishing them in the directory, in the rosters list, in the ministry lists, etc.
Sometimes Father will say " How come I don’t see Sally anymore at Mass?" And we’ll say, well, we heard she moved to Nevada, but she never said anything to us…maybe she did!

In a big parish, you can get lost in the numbers really easily. It doesn’t mean that the priest doesn’t care, or that he doesn’t notice you.
It boils down to courtesy.