I have dual L298N HBridge that is rated for 2A per line, like the one shown below. I also have DC motor that draws 2.5A, even more at startup. So I had idea to combine lines to split load evenly between them, like so:
Arduino to HBridge:
pin1 -> [ENA + ENB]
pin2 -> [IN1 + IN3]
pin3 -> [IN2 + IN4]
HBridge to motor:
[OUT1 + OUT3] -> motor +
[OUT2 + OUT4] -> motor -
Would this work, or that's secret formula for magic blue smoke?
The ancient, inefficient L298 should not be considered for anything but the smallest motors. Use a modern motor driver that can handle the stall current of your motor.
Pololu has an excellent collection.
even more at startup
As much as 5 to 10 times more. Look up the actual value.
Yes, your proposal will work to increase the current output.
However two plus two doesn't always equal four. The datasheet probably says that the maximum on one channel is only possible with the other channel undriven.
jremington:
The ancient, inefficient L298...
I know that now, but didn't when I bought it. I'm very new to Arduino and electronics. If/when it burns I'll get another part, have my eyes on a nice 43A driver.
jremington:
As much as 5 to 10 times more. Look up the actual value.
Actually can't, it's a car window motor from a scrapyard, with no readable markings.
MorganS:
Yes, your proposal will work to increase the current output.
Excellent, off to the laboratory! Will keep fire extinguisher handy 
Thank you guys for your help.
Actually can't, it's a car window motor from a scrapyard, with no readable markings.
Then use your multimeter to measure the winding resistance. Divide that into the motor power supply voltage to get the start/stall current.
I'll bet in excess of 10 Amperes.
Be aware that the doubling up trick won't work with stepper drivers like the DRV8825, it will probably blow them up... The L298N is a very simple device by comparison, just a current amplifier.
jremington:
I'll bet in excess of 10 Amperes.
For comparison, a little brand X door lock motor draws ~4A stalled.