I sent two transactions within 4 seconds of each other. The first took 16 minutes to confirm, the second is still unconfirmed 1.5hrs later.
Chances are what happens is that your second transaction used the change that was output back to you from the first transaction.
When you spend a coin that was only recently received (even if it came from your own wallet), then the bitcoin network treats that transaction different from a coin that is about a day or more old. It does this to protect the network from griefing that occurs. If it didn't do this, then a user could cheaply send funds back and forth from a single wallet at little cost yet at the same time impose a significant cost to the rest of the network in bandwidth and storage costs.
If that is what happened then a higher fee on the second transaction might have helped a little. Also, you can split up a coin yourself in advance so that when you do spend there are multiple older coins that can be chosen for spending without having to re-spend the change output from a recent transaction.
