Short answer: Unless you just want to see high numbers, I'd probably just settle for 2.5gbit and that should work just fine over Cat5. (It would definitely work over Cat5e.)
Can I bond them somehow to make them a single 5GbE port?I haven't bought a switch or router yet.
Yep. This is called "Link Aggregation" and quite a few switches support it. You'd need a switch on both ends to combine the two cables into a single link. Generally, it's more trouble than it's worth, both in terms of price and headache, but the option in there.
With the price of 10gbit equipment to do link aggregation, you might be better off running another cable. How accessible is the existing cable to make a Cat6a run next to it?
Short answer: Unless you just want to see high numbers, I'd probably just settle for 2.5gbit and that should work just fine over Cat5. (It would definitely work over Cat5e.)
Yep. This is called "Link Aggregation" and quite a few switches support it. You'd need a switch on both ends to combine the two cables into a single link. Generally, it's more trouble than it's worth, both in terms of price and headache, but the option in there.
With the price of 10gbit equipment to do link aggregation, you might be better off running another cable. How accessible is the existing cable to make a Cat6a run next to it?