It's just way better to fill as many tiles with rivers with one space between them for thickets and presumably forests, rocks, mountains and meadows. If it truly doubled the effect, the thicket should give 8% with two rivers and 16% with three rivers (if it doubles the already doubled value) or 6% with two rivers and 8% with three rivers (if every river just doubles the base effect). Why this feels weird is the description of the river tile which 'doubles the effect of adjacent landscape tiles'. They would also set the effective bonus to 0 and give a 4% bonus themselves and therefore an effect of +12% Attack speed. The second and third river would also notice a thicket next to themselves. This way the first river would notice a thicket next to it and set the effective bonus this thicket gives to 0 and the river gives the doubled effect of 4%. The way I think it works is the river doesn't double the effect of adjacent tiles but removes the effect from the tiles and adds the doubled effect itself. So the bonus is not multiplicative but additive. So I went even further and placed a thicket next to 3 river tiles and got 12% added. Placed a thicket next to a river and got 4% added as expected but when I placed a thicket next to two rivers I got +8% which surprised me because I thought they wouldn't stack.

Tested it with thickets (+2% Attack Speed). So tried the river tile and found it a little weird how it worked.

Loop hero river