No problems with mixing in same pin before injecting. It's not advised to let them mingle and degrade for more than 8-12 hours though.
Oddly enough, many companies now sell mixed ghrp/ghrh combos in one vial. DatBtrue is pretty adamant that this causes degradation and they should be in separate vials. He's the closest thing to an impartial authority in this area that we have. Just goes to show most companies will sell whatever they think people will by. Clearly, peptides like that should be best kept in separate individual vials. Market is kind of shady like that.