Arduino connecting multiple GPIO pins to one voltage line
Using multiple pins in parallel will share the current but there is a max allowed current per pin and for the entire mcu, you should keep the current within specs for both.  

The Atmel Mega 2560 datasheet, in section 31 (electrical characteristics), says:



but note that this is absolute max, so you shouldn't use that levels in normal operation. The normal operating current seems to be in the level of 20mA since that is the mentioned current in several parameters in the spec list and the graphs.

Also note that the pin voltage drops as the voltage you sink/source increases





I think you are better off using a driver for the Peltier cell, you can can use an IC, a mosfet or a transistor.