I would go for more 1 week externships rather than longer fewer . I did 2 2week externships and absolutely regretted it.
A- They suck, standing on your feet all day pretending this is the first 3rd molar extraction you've seen, while trying to make small talk without annoying the residents is hard work
B- Nobody really cares, as a resident now I usually talk to an extern for a few minutes, and already know whether we like them or not, we don't need 2 weeks to decide, if anything 2 weeks gives you more time to screw up, cause trust me; we probably won't remember if you did 1 week or 2.
C- Being a memorable extern for one week is much more valuable than being a passive one for 2 weeks, make yourself seen, and look busy; over two weeks people will start to realize if you're not always there wagging your tail like a dog (I'm sorry but it's true).
D- It gives you more to talk about on you interviews, saying you externed for 5 weeks, but only having 3 programs to talk about (probably one of them being your home program too), isn't as stimulating for conversation as having 5 different programs to compare and contrast against each other.
Just my 2 cents