Don't connect horns in series. They'll only get half the voltage that they get when connected in parallel, and neither will draw enough current to make any kind of squeak at all.
The reason one horn has 4 spade terminals and the other has 2 is simple. Have a closer look at the 4-terminal horn and you'll see that the 4 terminals are actually two pairs that are bridged together for use as a jumper terminal. You can see which ones are joined. You plug the +12V power feed to one of the bridged pairs, and the earth connection (via a switch or relay) to earth connects to the other bridged pair. That horn is now wired up to work. The two left-over terminals are simply there to run a pair of wires across to the two spade terminals on the other horn to connect that one as well. Note that it's a parallel connection.
Sometimes old horns will need to re-tuned. You do that by turning the tuning screw on the back in or out. Do one at a time so as not to piss the neighbours off too much. Put some ear plugs in. Hook a horn up to a healthy 12V battery, then turn the screw in or out as required to achieve maximum noise. There may be a lock-nut on the tuning screw. I can't remember. If you can't get one or both of them to emit anything more than a pathetic bleat, then it's time to take them apart and clean all the rust and dirt out of them.
If your horns have been apart before, there will be 6 x M5 bolts holding the front and back cases together. If they've never been apart since they left the factory, they'll be held together with rivets that you'll need to drill out and replace with bolts when you reassemble them. I can't tell from your photo whether those fixings are rivet heads or socket (Allen) head bolts with button heads. They look a bit like the latter, so your horns may have been apart before.
They're dead simple inside. You can't really fuck it up. Basically it's just a solenoid connected to the diaphragm and a set of contact points. Just clean everything you can find. De-rust anything that looks rusty and give it a coat of paint. (clean the contact points but don't paint them).
Stick it all back together with a smear of silicone around the edge to seal the space between the diaphragm and rear case. Don't bother trying to seal the front of the case. It's got bloody great holes in it for the noise to get out. Hook each horn up to a power supply and twiddle the adjusting screw. You should get a good lout honk from them. One horn should be a high pitch and the other low pitch.
