Is using diodes in this scenario a common good practice?
I would not do this as it complicates charging for example.

What I would do is balance the two batteries, that means discharging/charging  them so that they will have the same voltage and then connect them directly in parallel.

This balancing can be done by: 


connect both battery's - (negative) poles together directly
connect the + (positive) together with a resistor or (car) light bulb in between. A 10 ohms 1 W resistor would do the job.
Leave that connected for a couple of hours or a day and measure the voltages of the batteries. When they're almost the same you can safely connect the two batteries permanently in parallel.