So €205K left on mortgage and in 6/7 years time you want to build €700K house, and retain current house.
You are saving €4K per month, so estimate this stays constant as your salary increases, you have baby 3&4 and your spouse gives up work.
In 6 years you will have €288K cash, anything extra saved should go to paying down the mortgage. I see you got a nice uplift in salary so use bonuses etc to clear this to 0 in 6 years. Then you can get potential €500K mortgage plus your €300K savings to build.
It sounds doable, but you are probably not going to be able to maximise pension savings for you both at the same time. You will have assets of over €1M with both houses but if you become incapacitated, loose your job or any other disaster, it may all come tumbling down. Everything rests on you being a large earner for the whole family.
I am imaging you with a home in the city and a holiday home by the sea with you the wife and kids having a fab lifestyle in the city for term time and on/by the sea otherwise. Go for it!