My first thought is that you are always going to have risk with two obviously-old disks. There is also a mathematical increase in failure when you go from one to two disks, and when you say "would like to extend another 40G", I think spanning disks. If you mean combining them into one partition, I really would not do that due to the age of the disks since if on fails, you are suddenly in a situation where data recovery requires better tools since all the data is not just on one drive. It could be harder for you to recover.
Normally I would not think twice about having two disks, but yours are old, so be careful how you do it, and make sure you have any critical data backed up.