D&D 5E Using a Familiar for Second Concentration Spell?

[doctorbadwolf]

I would like to point out that the interior of the bag of holding is a pocket dimension. Putting the familiar in it should count as "on another plane" and break its ability to maintain the 2nd concentration spell.

most folks, IME, treat it like a bag in the sense that you can peak your head out of it and be on the same dimension.