For the amount of money that you would spend on a Time Capsule, you could get an AirPort Extreme Base Station and two external USB hard drives. One external HD could be connected to the AirPort Extreme, providing the common folder access between the two computers. The other HD can be connected to the Mac, providing the Time Machine capability. This allows for greater flexibility moving forward.
There are various reports of using the AirPort Extreme with Time Machine, both positive and negative. If you would like to try this route, first partition the drive using Disk Utility by attaching it to your Mac.
You can't easily partition a Time Capsule hard drive. Because it's on the network, Disk Utility doesn't recognize it and allow you to partition it. One way to partition the drive is to physically remove it from the Time Capsule, put it in an external enclosure, and plug it into your Mac by USB. Then you'd be able to partition it how you like and then replace it back into the Time Capsule.