Disconnect one and bury it in your backyard.   Seriously, you'll need to provide more details.

One for transmit, one for receive? Both aimed in the same direction? Limited space? Application - RFID du jour?

I'd assume they are very close to each other. Crosspolarized is the best quick solution if they're close to each other.

If co-polarized is required, orient them similar to laying out two pencils on a desk in parallel with each other, one atop the other, and not end to end. Space them apart as far apart as possible on a flat metal ground plane (desk) and add a long metal rectangular box between the two antennas. The box will prevent current from going over top of it since the E-field will be shorted out and the currents will have to go to the ends of the box and back to your other antenna. Current will also reflect off the ground plane edge and go around this box too.

Good results require good crosspole purity for your patches. This gives you some basic improvement. As VE1BLL stated, more detail is required, like how much isolation do you need and do you have a spade shovel or a gardener to help dig. It's always good to have more info.

Standard isolation numbers would be -12 dB if they're closely spaced and if you use the added box arrangement I mentioned and it was say 1/2 wave wide x 4 wavelengths long, you'd probably jump to the -35 dB isolation range. Crosspole and closely spaced can provide -35 dB, but you'd need to finely orient the two and maybe add some sort of cross pole tune-me-out screw posts between them to even better tune each other out.

kch
PS: Our company specializes in isolating two antennas very closely. We take the two antennas and and modify their nearfield polarization while maintaining good far field patterns to isolate them. I'm pretty sure we are the only ones to know how to do that. If that sounds like blatant advertising, sorry about the spam.

Sorry missed out quite a few details

One will be used as a receiver and the other as a transmitter. Both are horizontally polarized Operating frequency is 5.3 GHz both pointing in the same direction. trying to confine them within a area of 4 metre square.  

Each patch antenna is about 10 cm X 10 cm

And also I am trying to gain an isolation of about -60 dB or more if possible