Is it sensible to allow both grip modifications to be on one weapon?

If you're billing HPs I don't see why not.

Paying with HP ofc, couldn't see any reason to not so it myself but sometimes it is easy to miss obvious things.

Video game logic would probably say "only one grip upgrade," but narrative logic easily allows for you to put a pistol grip on a weapon, then customize that grip to better suit your hand. I'd probably just require you to move the attachments together if you got a different gun in the future, if I were the GM.

I don't get too terribly anal with these rules. Given the lethality of the weapons they've added in additional supplements, plus what can be achieved even with a non-min/max crafter, I see no reason to get too crazy restrictive.

Think of it this way. traditionally shotguns have the straight back grip as part of the stock. Upgrade it to pistol grip. Now choose the pistol grip that best fits your hand.