How to etch Double sided pcb?
I've successfully done etched double-sided PCBs using toner transfer paper.
This is not easy. Aligning it is a pain, but I recommend that you get some Kapton tape (because it can take the heat).

Make sure that the PCB you print out have some crosses (plusses) in each corner, so you can "easily" align them 'perfectly' up against the light. Note: This is not easy, because the toner transfer paper is slippery!

Start by printing the two sides out on the toner transfer paper (remember to print the bottom layer mirrored), then cut it, so there's approximately 1cm extra on all sides.

Now you make an envelope, where the printed sides will face the inside of the envelope. The envelope should be taped by using Kapton tape on for instance the left side and the top; only two sides.

Place the copper clad inside the envelope, so you now have a sandwich (don't eat it, though).

The envelope with the copper clad is placed on for instance a straight wooden board; make sure it does not bend in any way. Wood can take the heat we're going to apply.

Warning: The PCB will be HOT...
Turn on the iron; when it is hot, you push down hard for approximately 6 seconds (you count, you don't have time to look at the clock), then you lift the iron, turn it 90 degrees, put it down and push hard for 2 seconds, lift, turn 90 degrees, push down 2 more seconds and repeat this action just once more.
Using a tool, flip the PCB over and repeat the above action, subtract up to 2 seconds from the initial ironing if the PCB is still very hot.
Remember: The PCB is HOT, don't touch!

Pushing down hard means: Use all your body weight.
If the result is that the toner traces falls off here and there, then the timing is too short; you'll need to apply the push for a longer time.
If the result is that the traces are too wide/shaky, then you've applied pressure for too long.

Fortunately, you can retry by cleaning the copper clad, but you really want to avoid that, because cleaning it is a real mess and takes a lot of time.

Make sure you align the sides of the "envelope" perfectly, though. It really pays off to make precise work here, because when you put in the PCB, the sides are going to misalign a little bit already. If they're aligned perfectly from the beginning, then the error will not be too bad.

My description depends on how hot your iron gets, it also depends on the thickness of your copper clad (I used 1.6mm, the standard thickness, but also 0.8mm). You'll have to experiment a bit, before you go for the real thing.
I recommend making some simple printouts and simple envelopes containing 9 dots (top, middle, bottom, left, center,right), then you don't have to do too much cleaning while practicing.