If the boss will look at their IP history it may be confirmed or not.
This is easily avoided by using different VPN servers for each account, using one account on PC with landline/fiber and the other via mobile data, using one account only via Tor, using one account over public or neighbor's wifi, etc.
If the IPs are consistently the same then it proves it, but if they are different then it doesn't disprove it.
Exactly, this can prove if they are alts if they always have the same IP (also as you said it cannot prove if they are not alts due to VPN)
We have a chance to get the answer, but they seem overconfident about it so I think the IPs will result different between them.