I have a question about PhotoMOS relays like TLP3546A. Is it possible to connect two of these relays in parallel to increase Load current (upto 6A)?? I know there might be issues with Turn On and Turn OFF time
The turn on time between 0.8 and 5 milliseconds is a large variety. I would say this is unsuitable for parallel operation unless you perform further binning.
Worst case you can run most current trough one module for ~4 ms.
You can limit this to ~2 milliseconds by increasing led drive.
Don't parallel since the switching times can be too different.
Instead, use the optocoupler to drive a bigger transistor...or electromechanical relay. There's not much using both an optocoupler and a relay though since the relay already provides isolation so you could just use the relay directly (though you may need a drive transistor if your drive source cannot source enough current).