Splitting Cable Connection

I am trying to help a friend configure an Apple Airport Express for wireless
music streaming on Vista.

My first attempt at configuring the Airport at her home didn't work so for
my second attempt I brought her computer and the Airport to my place and
successfully configured the Airport ( eventually ) connected to my network
using my dLink router.

When I returned and connected to her Linksys router , it didn't work (
thinking about it now , I'm not surprised ...). Her router is not properly
configured anyway , so I'd like to reset it and start again...then
configure the Airport

I'm going to make another attempt , bringing her computer , her Linksys
router and the Airport home once again. We both use the same cable ISP and
the same Motorola modem provided by the ISP

My concern is that I don't want to connect my computer to her router ,
while I work on everything , for fear of screwing up something on my
network including my wireless printer.

I could disconnect my dlink and just connect her linksys to my modem and go
from there , but as this will probably take some time , I'd also like to be
able to use my own computer at the same time. I wondered if I put a splitter
before the modem and then sent a line to my dLink and her Linksys , would I
able to be work on both at the same time without any major issues ? ( apart
from perhaps a speed reduction ?)

I hope this isn't too confusing...
TIA