Could a decker use multiple cyberdecks, each running an Agent, to gain teamwork test dice from more than one agent?
There is nothing in the rules that prevent this. It is expected that the high nuyen cost of cyberdecks provides a significant enough barrier to prevent some enterprising decker from amassing a digital army. Functionally it's no different from a Technomancer being assisted by his Sprites, or a Summoner being assisted by his Spirits. The main difference is Summoners and Technomancers can call forth increasingly smarter and smarter assistants (higher rating Sprites/Spirits), whereas you will always be limited by a maximum rating of 6 for your Agents.