```python
order.debit(customer1, 50 customer2)
# ... 
# default funding source for customer1 changes
# ... 
order.debit(customer1, 100, customer3)
```

i was also going to propose:

```python
order.debit(customer1.sources[0], 50, customer2)
order.debit(customer1.sources[1], 100, customer3)
```

in case you wanted to control the source.