Isuzu appears to be fairly reasonable when it comes to the IMGs. They seem to recognize their snafu and are willing to do the right thing (e.g. the previously mentioned doing the work after the warranty period for some folks).

Surely your dealer will know that if one gasket of the same material and design fails the other cannot be far behind. This is all the more true since this is a widespread COMMON problem. If they are at all service oriented they should just do it without you even asking. If they aren't planning to, then ask and point out the upcoming need and obvious cost savings of doing it now rather than later. If that doesn't work call Isuzu and tell them what is going on and see if they don't agree that it will save them money to pay for both jobs at once rather than one at a time. Like I said, based on other posts IMGs seems to be a case where they will do the right thing. In any event I don't see what it can hurt to at least ask.