Hello every one

This is a simple diagram of my 2 sites topology.
I'd like to use an IPsec tunnel to connect the two offices.

I know how to do the tunnel (e.g. Peer,Policy and Proposal) but what I do not know is that which Private IP ranges should I use when creating the tunnel)

Can I change the setting of each site in a way that I only have one IP range? for example 192.168.1.0/24

Since I'm newbie to this stuff please be a little more specific.
Thanks in advance.

The right side is site 1.

ADSL Modem gets a public WAN IP from ISP and then gives a private IP to the Router. The router then gives another IP range to different clients.

IMHO:
1. Switch ADSL modems into transparent bridge mode (by yourself or ask provider) - your benefit will be full control with mikrotik
2. IP addresses ranges must be different on sites. Use routes to connect both lans. It is possible to use same ip range on both networks but not conventional for slow networks (not optical internet) - broadcasting and network productivity isues too much non useful trafic.