I use Combustible Gearhulk as a 1-of in a casual deck with 10 other cards with a converted mana cost of 6 or higher. If they don't let me draw 3, it could kill them outright.
Torrential Gearhulk is a card that is similar to Snapcaster Mage, and at the same time, altogether different. You'll want a suite of midrange or high cost instants in your graveyard. You can use him as card advantage, or make a graveyard deck that fills your 'yard with silver bullets to get a free spell that's very effective in a given situation. Gifts Ungiven and some modal spells like Commands and Charms might be worth looking into.
Insofar as using them together, maybe 2 or 3 of each as finishers in a U/R/x instant-heavy control or counterburn deck?
If you want to use both in the same deck, you probably want to be playing a UR spells deck with big flashy spells. High-cost spells in UR take advantage of both Gearhulks -- lots of damage with Combustible and good targets for Torrential.
You probably want to play Brainstorm, because you can use it to stack your top 2 with high cost cards just before playing Combustible Gearhulk, guaranteeing free cards drawn or major damage dealt.
Obviously don't play all at once, but you can use a few of these as the core of the deck and then use some library manipulation like Brainstorm to set up Combustible. Torrential will just get value from being able to reuse expensive spells for free!!
It depends on whether you're in 1v1 or multiplayer, but there are definitely ways to build decks that can take advantage of both Gearhulks.