GW pots are typically only half full, so if the mix you want is 50:50 you can just pour one in to the other and be done with it, though you'll end up with a rather full pot of paint which might be hard to shake. However I've done it in the past, I poured a pot of bleached bone in to a pot of Deneb Stone and used that as the base for my Tyranids.
Typically though, I just count the number of brushfulls of paint I use to get an approximate on the ratio I need and then mix it when I need it. Though you need to be careful when you mix paint every time you use it, as over time the paints will start to dry out, probably at different rates, so if you mix 1:1 now, it may not look the same as 1:1 mixed 6 months from now or mixed from new pots of the same colours.
If it's mission critical that the paint is identical from one model to the next, I'll either mix large amounts in a mixing pot or just pour one paint in to the pot of another. I made my "man flesh" colour by pouring a pot of dwarf flesh in to a pot of elf flesh
But be aware,
GW aren't consistent with the amount of paint they put in a bottle. So just look carefully at the paint level to see whether it is half full or more than half full and if it's more than half full, don't pour the other paint in to it as it'll become too full by the time you pour the other paint in to it

I assume
GW are honest and the volume of paint listed on the bottle is the "minimum" they ever pour in to a pot, coz I've seen some pretty large variation from one pot to the next.