Certified GKH Gardening Expert
It is possible to plant these two plants in the same or a similar mix because they have similar potting requirements. However, you won't be able to fertilize your cactus if it's in the same pot as a Venus fly trap, so it's best to have them in separate pots.
Venus fly traps need very nutrient poor potting medium because they derive most of their nutrients from the bugs they catch and excessive nutrients can harm them. You can use sphagnum moss or peat moss mixed with sand. Here's more:
Cacti need a well drained potting mix, potentially containing the same ingredients as the above. Or purchase one that says "cacti" or "cacti and succulents" on the label. Here's how to make your own: