AirPort Extreme base stations always forward frames between wireless clients. It is on by default and cannot be turned off. If laptop A can't see laptop B, your problem lies elsewhere.
Maybe multicast isn't working reliably, so ARP and/or your discovery protocol (Bonjour? NetBIOS Name Service?) isn't working reliably. Perhaps one of your laptops has a wireless card that doesn't handle multicast correctly (multicast handling can be tricky on 802.11 with certain wireless security combinations). Or perhaps you've set your multicast rate on the base station too high, and one or both of your laptops can't reliably receive multicasts at its current location.
As a temporary test, try disabling all wireless security on your base station, and setting its multicast rate to the lowest setting. See if that makes a difference. That should indicate whether or not you have a multicast problem.
If you still have the problem even with those settings changes, you probably don't have a multicast problem. You might have something set wrong on the personal firewall software on one or both laptops. You could try cabling up both laptops to the LAN ports of the base station and seeing if you can connect that way, to further isolate this problem from wireless.