Hi jonparadise2,
Thank you for your question. This is a good one!
There are several ways you could go about this. You could use conditionals, or segment variables, or search.
I like the idea of using search.
So the idea would be to use something like a radio or a select drop down. I like the select drop down idea, so I will use that. Check out this screen shot http://cl.ly/image/2M0t3C3y1S0e. This gives your user the option to post to Blog A, Blog B, or both.
Then in your channel entries tag use the search:field -
search:blog_selection="BlogA"
That should give you an idea of where to start with this and how you might get creative in implementing it.
You can really creative and use something like the P&T Field Pack to spruce up the selection in the Control Panel.
Let me know what you think!
Cheers,