A landscape planting

I want to use a shallow 17x24 pot .I was thinking of dividing the potent 2 sections with a river thru it One side [ call it A] it has rock formation with 2 tree I have not picked any type of plant yet . On the other side [ call it B] a isolated desert scene with several trees very flat. My thought is a flat top tree by itself A river separates the two sides.
Side A I was thinking a deciduous tree to show fall color
Side B i was thinking a Kings-boxwood .

I was told not to introduce two different types of trees to the same pot? b/c they would require two different watering problems

My thinking is the river could divide the watering needs between the two types of plants

Any ideas for the side 1 for the deciduous tree / as the isolated tree with be a Kings-boxwood ? thank you Bill D