Do you have just one lighting circuit, coming from the CU, or do you have 2 MCBs or fuses, one for upstairs and one for down?
And there are a number of ways in which 2-way switching can be wired, and how yours is done will affect how you would go about making the change you want.
(I'm fighting the urge to ask why on earth you should want to do this though, why, for example, if you get up in the night for a pee you would want the hall light to come on when you turn the landing one on.)
My suggestion is that you look at the info here, in the Wiki and the For Reference topic on how 2-way switching is wired, understand how they work, determine which one you have, and work out for yourself how you'd wire the new switches.
Then post a drawing here for people to check.
It will take longer, and require more effort on your part, but you will learn, and with any electrical work you should only do it on the basis of a genuine understanding, not a wiring-by-numbers approach of "put the blue wire into L2" etc without a clue as to why.