Splitters allow you to connect multiple phones to the same line. They do not magically give you two lines.
Setting up a DC-PC server is a pain. You need to download an installation of Linux and software for running a virtual OS inside your real computer (like VMWare), and then you need to do a bunch of configuration on the virtual Linux machine. Unless you have the right modem (revision A, IIRC), you also have to buy components to build a line-voltage simulator, because connecting your DC directly to your PC won't provide it with the same voltage that a real phone line would have, and some modems won't work without it. It's also necessary to buy a voice modem for your PC, since it's unlikely that even if by chance your PC has a modem that it's the kind that accepts incoming connections. You also have to start your computer and set up the virtual server every time you want to play, and you have to manually tell your computer to answer the DC's phone call every time PSO tries to connect, because apparently even Linux isn't smart enough to notice and answer automatically when the DC is dialing.
Even if you do all of this correctly, you may still be plagued with lag and frequent disconnections that make the game unplayable. Quite simply, DC-PC servers are for people who are either too cheap to buy BBAs or want to play other DC games online that don't support the BBA at all. BBAs do cost quite a bit, but you're better off saving money for however long it takes and buying one than you are trying to klouge together some strange system with coathangers and toasters to get your DC online.
...You could also just use PSO PC, which avoids both the cost and hassle associated with getting a DC online.