Maintenance for the week of July 13:
• PC/Mac: No maintenance – July 13
The issues on the North American and European PlayStation® megaservers have been resolved at this time. If you continue to experience difficulties at login, please restart your client. Thank you for your patience!
Sorry for asking the obvious, but you did equip the second weapon? You obviously can't swap weapons with only one equipped. I've never had any issues (other than lag) barswapping on PS4.
If you've already got weapons equipped in both bars, changing your keybind may also work (if that's possible on PS4??)
Had a similar issue on PC and that's what solved it..