I also have an xSIID and an xDF3 that are 16 mm apart (on either side of a vein) in my left P0. My xSIID was my first install, and it was kind of a non ideal place (closer to the bone than I wanted?) but it worked out because it hasn’t bothered me or caused problems and left enough room for a flex chip that I recently (friday) replaced with an x series.
I do a lot of farm style work, sports, etc … and mine have held up great. I have whacked my xSIID with a hammer several times. Still functioning as good as the first day I installed it 3 years ago.
The only thing I learned the hard way is that you don’t put flex chips there! They fail fast in that location!
Maybe you could use some pre wrap to “hold it in place" if you want some extra comfort? I don’t know that it will actually help, but it might make you feel better mentally lol