I'd be extremely cautious of splitting this summer. It's ultimately your choice, and individually, splitting may make sense in your case based on your priorities. But I'd be worried that a firm could interpret splitting as a lack of commitment to either the firm or possibly a geographic area. Another problem is that some attorneys may not feel like they had enough time to get to know you, and would be less likely on that basis to vote in favor of an offer.
In times where firms may be looking for a reason to no-offer you, either of the above may really count against you. It's a gamble on how well you can play the game over the summer by figuring out who in each department "matters" (and it's often NOT the partners), really giving those people a chance to know you, and talking up your commitment to the firm/area in such as way as to be convincing. It's a balancing act - if you don't talk it up enough, they think you're just there for the summer fun; but if you talk it up too much, and then decide to turn the offer down, you'll risk burning a lot of bridges.
(but if it's your 1L firm, I don't think it matters as much. The few weeks at the end of the summer really come across as more of a courtesy to the other firm, if nothing else. It probably won't give an impression of a lack of commitment or eat into your time at another firm too much)