The DIR-657 supports VPN passthrough. So in other words a VPN client behind the router can connect to their corporate network with ease. This also includes multiple outbound VPN connections using PPTP and IPSEC. Now if you are connecting to the same location, this can create issues. Connecting more than 1 VPN connection to the same endpoint + same user account is even more unlikely to work properly. Most VPN endpoints either do not accept more than 1 connection from the same origin and some will not accept more than one concurrent user with the same credentials. Both outside of the scope of the router.
What you could do is if your employer allows you to create a site to site IPSEC tunnel with them, you could purchase a VPN router and create a 24/7 connection. This would allow more than one computer to the corporate intranet without issues. This of course requires them to be on board with such a proposition.
If you were connecting 2 VPN connections to two different endpoints (2 different companies for example), the DIR-657 should have no issues whatsoever. Just make sure the IPSec and PPTP ALG's are checked under firewall settings on the router. They are already enabled by default.