You would lose 4gb of memory on the 8gb if you crossfire and they will both run at the speed of the slowest card.
There are very few games that work or even really benefit from crossfire. Both AMD and Nvidia have abandoned Crossfire and SLI.
So unless you have that select few games that will benefit it likely is not worth the trouble. Also know that you likely will have to run very old drivers as it has been a long time since crossfire worked right in the drivers.
This is the list of compatible games, it doesn't mean that they all work as many have lost support by the developers and or in the AMD drivers. Also the performance gain on the vast majority is minute. Very few games were ever optimized enough to have really good crossfire support, but there are a few.
https://amdcrossfire.fandom.com/wiki/Crossfire_Game_Compatibility_List
Unless you already have 2 cards I highly recommend you don't buy a second. Put you money toward a single card solution that meets your needs. Support is discontinued and only gets worse from here.
