What? You'll be transmitting from one radio right into the other. I don't know if it's designed for that. You'd need serious notch filtering or cavities. This page says it's for radios that have two antenna connections to use one antenna. Some radios have two connectors so you can have individual band antennas and manually switch over when you change bands. It looks like this will enable you to use one antenna going to both connectors on the one radio. Read here.
No, you won't be "transmitting from one radio right into the other", since the diplexer splits the frequencies going to each port and provides isolation between them (according to the Diamond site, this one provides 60db isolation). That's what a diplexer does.
In general a diplexer is used two ways:
1) To combine two antennas (tuned for different frequencies) for use by one radio. So you could connect a 2m antenna to one port, a 70cm antenna to the second, and a dual-band radio to the common port. The radio will "use" the antenna tuned for the appropriate band when you transmit and receive.
2) To use one antenna for two radios. For example, dual-band antenna on the roof connected to a 2m radio and a 70cm radio.
What you describe is a special case of #2 where you have a dual-band radio with separate connectors for 2m and 70cm.
There are also triplexers that split signals into three bands. I have a tri-band antenna on my roof connected to three different radios, one for 2m, one for 220, one for 70cm. You can safely transmit on any of them without affecting the other two.
As has already been stated, different diplexers and triplexers have different band splits and you have to pay attention and get the right one for your need..
