Adding a second T1 will certainly double the amount of bandwidth you currently have. However there are other pieces of the puzzle that you should take into consideration that may be causing you lag time as Internet traffic has increased. I would ask the question of; What is your T1 used for. Internal employees accessing the Internet. Do you have web servers or something to that effect that external clients are connecting to.
If it is only for internal employees that are accessing the internet. You may want to see what they are doing. Non business internet activity can quickly eat up your available internet bandwidth if a number of employees are accessing video new feeds, music videos and the like.
The other things you should check into is your Firewalls and Routers. Are they still sufficient enough to handle the increased loads you have experience. And if you are serving up web services are the like to the public, are you webservers up to snuff to handle the increased loads.
A combination of a small percentage of a problem in each of these areas could equate to a significant lag time in your internet access speed, either from internal out or external in.
The Humble Network Admin