x2 vs x4 SSD limiting question

[derpy]

I'm using 2 Evo 970s, which are spec'd for x4 - but the motherboards I could find only have 1 x4 & 1 x2. The issue is they're in a mirror mode - so the performance will be bottlenecked by the slower drive. What is recommended in this case? Do I need an expansion card in order to use two x4 spec'd SSDs?

P.S. The CAPTCHAs on this forum fail often.