Although that's technically called 'Dual Agency' because listing agreements and purchase contracts are really signed with a brokerage (with individual Realtor representing the brokerage) rather than the specific Realtor a buyer/seller work with; working with two different agents in the same office is actually, very common. Each individual agent will have his/her fiduciary duty with the specific client they are serving.
This is very different from the true 'Dual Agency' where one agent represents both buyer and seller - the water can get murky that way.
For example, my company is the #1 real estate company with hundreds agents and 27+% market share in Marin. We have lots of listings and tons of buyers represented by Realtors from my brokerage. If we are not allowed to accept offers from clients represented by other agents in my company, it's be a very sad situation for our clients, whether sellers or buyers.
Many times we accept other company's offers over our own office's offers - we only do whatever is best for our clients.