treeA = [...]
sumA = ...
treeB = [...]
sumB = ...

// add values from a to b
For Each a in treeA
  If sumA > sumB AndAlso Abs(sumA - sumB) > Abs(sumA - a, sumB + a) Then
    treeA.Remove(a)
    treeB.Add(a)
    sumA -= a
    sumB += a
  Else
    break

// add values from b to a
For Each b in treeB
  If sumB > sumA AndAlso Abs(sumA - sumB) > Abs(sumA + b, sumB - b) Then
    treeB.Remove(b)
    treeA.Add(b)
    sumB -= b
    sumA += b
  Else
    break

// if we still have time try a 1 for 1 swap
For Each a in treeA
  For Each b in treeB
    If Abs(sumA - sumB) > Abs(sumA - a + b, sumB - b + a) Then
      treeA.Remove(a)
      treeB.Remove(b)
      sumA = sumA - a + b
      sumB = sumB - b + a
    End If
  Next
Next

// depending on time we can try adding single values from a to b and/or b to a again
// another option is to try a swap 2 from a with 1 from b and vice versa, once you start with combinations Pick 2+ your running time will increase substantially