Migrating one site into a subdomain of another. Good or bad idea?

Hello,

I have a client with two websites. They both deal with the same topic but one has more content than the other. They would like to do is leverage the content from siteA (which has less traffic but a long history) into siteB.

They propose creating a subdomain: siteB.siteA.com

I am not knowledgeable enough to provide good advice and I'm hoping some of you might be able to help.

Is this a good idea? Will they be punished by google?