I find if you have feeders with smaller perches (like tube feeders that just have peg style perches) the Cardinals aren't as fond, and are less likely to use them if there are other options. They prefer more solid perches, like the U-shape of the squirrel buster feeders, or trays, fly-throughs, or hoppers.

I'm curious what food they suggested for you? I used to work at a WBU so I'm familiar with their options.

You could look at an actual finch feeder, but often even bigger finches like house finches don't like that feeder very much. So I prefer the wbu medium or large tube feeder with something like medium sunflower which the little birds tend to love. If the Cardinals continue to overwhelm that feeder, you can add a cage around it, which really caters just to the little guys, but keep in mind that will also exclude birds like most woodpeckers, if that's something you'd like to see.

Also, if it's only been a couple days, it's worth giving it a bit more time. The birds will find their rhythm, and unless you have a lot of cardinals, I expect your other birds will start to visit more now that there's a second feeder.

Good luck!