"2 Cable modems, 2 separate IP's, one line-MAJOR LAG!!"

Was wondering if you folks could give me your opinion on something. I am a Comcast customer. (I know, I know but it is all that we have here)
I have two separate cable modems hooked up. One is a Surfboard 4100, the other a Surfboard 3100.
We have one line into the house, completely separate from the cable that feeds our televisions. That line is split with a Comcast splitter (supposedly the "correct type") with one line then going to one computer with the SB3100 and the other going to another computer with the SB4100. I pay for two separate IP's.
I only pay for having one modem hooked up tho as
Comcast couldn't provision the SB4100 with two different IP's. (@Home could with no problem whatsoever) They are letting me use the second modem at their request and with their advice because of the problems on their end. But here is the problem. One day the connection on this PC is awesome and the other PC is in the toilet. The next day the other PC is rockin and rollin and this PC is terrible. Then both PC's suck, then one again and the other and it goes on and on. Then no problems for days. Then it starts again. I can play an online game and my ping is awesome and the PC in the other room is connected to the same game and the lag is so bad you can't play the game. Then vice versa. When one of us hosts and the other tries to connect, it is almost impossible to play together. It doesn't matter who connects first or last or anything like that. Have spent hours on the phone with Comcast and of course, no help whatsoever there. The line has been tested numerous times and they always say "you guys are awesome out here, no noise, etc etc." Both PC's are very similar in CPU, HD, Ram. What in the heck do you think a problem like this is?

First of all you could have just one modem and a SOHO router and connect any number of PCs.

Any way, tell these guys to isolate the two cablemodem lines. That means put a filter between them to avoid spurios transmission to leak into the other line.

Disconnect (completely) one cablemodem and navigate with the other one. Access the internal web page and see the transmission values. Reconnect the other cablemodem and repeat.
Do the values change?