Two Netware servers on the same 192.168.1.x schema

Is there a reason they are in two separate trees?

Are the clients configured for both IP and IPX? Generally speaking, they should not be.  Set them up for IP Only if possible.

IPX is actually a better protocol than IP for local area networking.  It is much easier for the client to find services using IPX/SPX than with TCP/IP, especially in a situation such as yours where you have two servers, each in separate trees, on the same IP subnet.  NCP over IP prefers SLP for initial service discovery, especially if you don't have DHCP pushing your DA address.

Do you also have two separate SLP DA's with separate scopes?  If so,  you'd have to make sure your client lists both SLP DA addresses in the Service Location tab.  Usually there's only one SLP DA in a subnet, and you should only have one SLP DA servicing a scope.  If you have two SLP DA's or are running unscoped (which isn't recommended and I don't think is supported in 6.5) service discovery can be confused.

Do you have DNS running?  For best results, I've found that, lacking DHCP SLP service type records your next best option is to make sure both the server name and the tree name have A records pointing to the server's IP address.

But, first and foremost, I recommend you take IPX off the client.  Multi-protocol client configs tend to confuse things.  If you have to leave it on the server, fine, but take it off the client.  Even if you need to use IPX for ancient print servers you don't have to use IPX on the client to communicate to the server - unless you're mired in the old queue-based service instead of NDPS.  If you must have IPX on the client, make sure TCP/IP is preferred, and make sure you address your SLP issues.  Also do the DNS trick I mentioned in the previous paragraph - or put entries in the local HOSTS file on all the client PC's for the tree names, associating them to their server's address.  

The best way, IMHO, is to stop with the separate trees.  If there's no logical reason to have separate trees, you shouldn't.  Merge the trees, with the 6.5 server's tree as the target tree, so it has the tree's CA and is Master of Root and is SLP DA with a single scope.  That way, you can have one tree, one set of user identites to maintain, one point of authentication - and you can have at least one level of redundancy for eDirectory.   The licensing differences between 6.x and 5.x don't preclude them from living in the same tree.  The main thing is whether or not you kept up with updates for 5.1 so it has the latest version of eDirectory that can run on 5.1, and didn' t update your 6.5's eDirectory way past that level, so you don't have any schema issues.   If a user only has rights to services on the 5.1 server it won't grab a 6.5 license, even if it uses the 6.5 server as its login server.  It will have an authenticated session (the one without the asterisk) but not a logged in session on the 6.5 server.

Of course, merging trees can be a lot of work, so just take it as unsolicited advice, and look to the other things I mentioned first.

The netmask you use will drive what IP range you can use.  192.168.1.x with a 255.255.255.0 netmask can only see 192.168.1.1-255.  Using a mask of 255.255.0.0 will open up all of 192.168.0.0-192.168.255.255 (officially, it's called "supernetting").   HOWEVER -- although NetWare 6 (and probalby 5) can do supernetting, your router might not be able to.

I still believe that the problem lies with a combination of the clients not knowing exactly which protocol to use, and SLP confusion because of the two trees.  Given the hand-off nature of the 5.1 server, and the need for IPX, I still recommend installing IPX on the NetWare 6 server.  You might also consider then un-installing ans re-installing the NetWare client in IPX only mode.  Leave Ip on the workstations for internet access & so forth; having the NW client do only IPX will not affect that.