QSFP28 - is there something like a crossover cable?

There's no old-style crossover patch cord or whatever. Properly working passive copper (You don't use optics, do you?) QSFP link should work regardless of you switch Vs switch-less use. Are you using one of those or is it anything third-party?

http://www.mellanox.com/products/interconnect/ethernet-direct-attach-copper-cables.php

Reference Mellanox thread (unanswered so far).

https://community.mellanox.com/thread/4125

QSFP28 ports are all alike (just like SFP, SFP+, ...), there's no MDI vs MDI-X pinout like for twisted pair. Therefore, straight cables aren't used at all. DACs have an integrated crossover, there's nothing to worry about.

Possibly your DAC isn't compatible with the NICs, some are very picky about the branding. It's also possible that the NICs aren't configured alike. You should check the driver messages on why the link doesn't come up.