I'm not at my Lightroom 'puter right now, but I'll toss out some thoughts.
The first thing you can try is to change the Capture Time in the image exif of the "bad" camera files. LR3 has the ability to do this and to actually write the time into the exif. Give that a try. Search Help for Capture Time and see if it turns up the needed info. You can change the time on a batch of images and have it make the changes in increments you specify, if I'm correct. If it works, you can have the Sort Order by capture time and then it'll be easy to rename them in the proper sequence.
If that doesn't work or you need another approach, you can go manual:
First, is there any difference in the file names from the two cameras? Or, if using the original filenames from the camera, there must be a differece in the image numbers, right? So if you sort to get them in sequence by image name/number, then you could set the LR Sort Order to User Order (I believe that's what you do, using the Sort tool -- you could find details in help). Then, it's up to you to drag images in the film strip into their proper order -- I'm thinking LR can't help you there
. From there, do a test of a set of your images -- select a set, rename them and see if they behave the way you would like. But the capture time will remain the same you you would have to have the Sort Order do do it alphabetically.
Now, that won't