Can I use 2 harddisks to backup using time machine (not duplicate, but to extend)
Yes - there are two ways to do this.

Apple intends and designed Time Machine to work with disks in a sequential manner. You are expected to fill up the first disk and put it on the shelf. Then attach the second and fill it up. You can then go back to the first and this lets you have more history before the oldest backup is reclaimed due to space running out.

An alternative is to use Mac OS X built in RAID functionality. You will have to erase all the disks when you put them into a RAID the first time - but you can have two disks with half the data written to each every time a file gets written. This is striping or RAID 0.

Using a RAID would mean both drives need to be connected all the time. You would then have one large backup and since Time Machine won't be saving two copies of many files - you will get much longer time for storage using a RAID than the first method of using the drives sequentially. If your boot drive is almost full - I've seen times where the first method could store a month or two of history, but years would fit on the second method. It really depends what files you have and how often they change.