asBehaviorSubject

import rx;

auto num1 = new BehaviorSubject!int;
auto num2 = new BehaviorSubject!int;

BehaviorSubject!int sum = combineLatest!((l, r) => l + r)(num1, num2).asBehaviorSubject();

assert(sum.value == 0);
num1.value = 10;
assert(sum.value == 10);
num2.value = 20;
assert(sum.value == 30);