|What colors of inks are in the two cartridges?|
If one is a color, or three colors, the other black, and the one that uses more ink has black ink, if you don't print much in color, you're going to use up the black ink faster.
Some Canon models that have more than two cartridges have two black ink cartridges. They're actually ink tanks - the print heads are built into the printer or all in one, not the cartridges. A friend of mine had a model with 4 CLI-8xx cartridges, 3 colors and black, and a PG-5 black, twice the size - I was the one who took care of the cartridges for her. The smaller black one is apparently not used for printing text and most graphics - it's used only for other things such as when printing photos. If you have more than two cartridges, one is twice the size of the others, the larger black one has pigment based ink, all the others have dye based ink.
I used bulk ink and a chip re-setter with the cartridges for her printer. If you wouldn't mind doing that to save some money, possibly getting your fingers inky, I can tell you more.
Otherwise, rather than buying new Canon cartridges.....
- you can buy original Canon ones that have been refilled by pros on the web for as little as half the price or less, with the chip already re-set.
- or - many local places that refill cartridges can do the same with your empty cartridges.