1 signal two stepper drives

[JAZZCNC]

It's a usb input

Ok that will be why it can't Slave and this alone is sign that it's not a very good controller, expect issues along the way from other things.!

I can use a seprate power supply to power the drivers so I don't need to wory about current problem.

The current I'm talking about has nothing to do with the current required to power the drives and motors so PSU size doesn't come into it.

Without getting too deep or technical I am talking about the current Step & Dir Inputs which are going to draw from the controllers Outputs. Each Output will have a max amount of current it can supply which will be in the Milliamp range and the drives Inputs will require a certain amount of current to operate, again in the milliamp range, which they draw from the Outputs.
So, for instance, let's say your controller's Output can provide 15ma and the drive's input requires 7ma to operate so when operating a single drive there is no problem because you have plenty of spare capacity. However, controlling 2 drives then you do have a problem because each drive will pull 7ma requiring 14ma total but the output can only provide 15ma which doesn't leave any spare capacity and you could get issues from signal loss and lost steps, etc.
If the inputs pull more than the Outputs can provide then you could damage the controller and you will definitely have a signal loss at best if they work at all.

So to know if this will work for you then you first need to know the spec of the controller's Outputs and the spec of the inputs of the drive.
If not then you could easily run into trouble with crazy shit happening or blow the controllers Outputs.

Now because I'm guessing it's a cheap controller because its USB and doesn't slave and the fact the drives will probably be cheap drives to match then there is a good chance you will have the worst of both worlds where the Outputs are low current and drives inputs will require a high current. This is common of cheaper components.
In all honesty, USB is shite for CNC as it's flaky in power terms and easily affected with electrical noise, best avoided if possible.

I could just square up the router after every job done, I have asked other people and they've said they've never had a problem doing it this way

Yes you can and some people just crash the machine into hard stops to square up but why would you when a proper controller will and should square it up for you in nice controlled why without you ever having to think about it.?