Ok, I understand but just as general info: when you buy paint,straight poly,or stain thinners are used in the process to make gloss to semi gloss more thinners to make satin -matte to eggshell ect...
The mineral spirits wont do any harm or give you inconsistent results it will just break down the poly but only when wet.You can use a mason jar pour some of you 2 in 1 and add mineral spirits @ 3 to 1mineral spirt it'll make life alot easier.This is if your doing more than one coat though.If your doing one coat or on you final coat use the stain poly straight with no thinner added.
Wish you the best with what ever you decide,just passing along some thoughts.