They get added, here is a toy example, where I have grouped two symbols: add and mult. I pass data through them and get the gradient back:

```
# input
a = mx.sym.Variable('a')
b = mx.sym.Variable('b')
# ops
add = a + b
mult = a * b
# output
out = mx.sym.Group([add, mult])
# bind shapes, get executor
executor = out.simple_bind(mx.cpu(), a=(1,3), b=(1,3))
# data
a_data = mx.nd.array([[1,2,3]])
b_data = mx.nd.array([[3,4,5]])
# Forward pass
output = executor.forward(a=a_data, b=b_data, is_train=True)
# Backward pass
head_grad = mx.nd.ones((1,3))
executor.backward([head_grad, head_grad])
print(executor.arg_dict)
print(executor.grad_arrays)
```

```
{'a':
[[1. 2. 3.]]
<NDArray 1x3 @cpu(0)>, 'b':
[[3. 4. 5.]]
<NDArray 1x3 @cpu(0)>}
[
[[4. 5. 6.]]
<NDArray 1x3 @cpu(0)>,
[[2. 3. 4.]]
<NDArray 1x3 @cpu(0)>]
```

We have:

d(add)/da = 1

d(mult)/da = b

d(add)/db = 1

d(mult)/db = a

We see that effectively our grad array for `a`

is:

`[1+3, 1+4, 1+5] = [4, 5, 6]`

and for `b`

is:

`[1+1, 1+2, 1+3] = [2, 3, 4]`