I have a 45 liter capacity oven toaster grill, with 3 rack levels. Can I use the 2 shelves at a time, to bake 2 cakes simultaneously?
It will most likely not work "just like that". Even if your otg has both top and bottom heating elements, the cake on the upper rack will have the heat to its bottom blocked by the cake on the lower rack, resulting in an overcooked top and undercooked bottom part, and for the cake on the lower rack it will be the other way round.
You can try to swap the two cakes after some time (probably between half and two thirds of the baking time) to compensate. But opening the oven door partway through the baking (thus letting hot, humid air out and colder, drier air in) might itself negatively impact the outcome.
If your otg has a convection fan, that is probably your best bet. The circulating air will distribute the heat much more evenly onto all sides of both cakes.
To convert a conventional recipe for convection baking, you typically reduce the temperature by 20°C, maybe put a bowl of already boiling water in the oven along with the cake (to make the air more humid and thus compensate for its movement, which would otherwise evaporate more water from the cake, causing it to dry out), and start keeping a close watch on the colour and texture of the cake's surface about 10-20 minutes earlier than the recipe says the cake will be done.