On paper yes, in practice no. The RX 480 is a good 75% faster than the 7870, so theoretically two 7870s is 25% faster than an RX 480, but scaling is very rarely linear, ranging between 0% (for games which do not support multiple GPUs) and 100% (for extremely well optimized games, such as Ashes Of The Singularity), and there are artifacts of multiple GPU setups which can detract from the experience (microstuttering, for example.) Also, the 7870 has been discontinued for some time, and it's unwise to purchase a used GPU given how many were ruined by cryptocurrency mining.
So my advice would be to purchase an RX 580.
