Definitely safer to send them separately so you receive 2 separate tracking numbers, covering yourself for any possible item not received cases, and also allows payouts to be released sooner.
Combining postage is still an issue with Simple Delivery, still not possible.
Some sellers are leaving a message on their listing, for example..... ' If you wish to buy multiple items, please let me know before buying, I will relist them as one listing so only one postage cost is charged, combining postage is not currently possible. '