This does require that the valve have a parallel gallery, but now days most multi-spool valves have this. If yours dosn't you will have to replace it with a valve that does.
When the flow is split in two directions, such as to lift the loader and curl the bucket, the fluid will flow to the lowest load/restriction first and move that cylinder first. For instance if you are trying to lift and curl a bucket full of dirt, the curl is probably the lesser hydraulic load compared to lifting the bucket up, so most likley the bucket will complete the curl and once the curl cylinders bottom out, then enough pressure will be built in the lift circuit to lift the loader arms. The same goes for dumping and lowering a full bucket. The loader arms having the greatest load and the least resistance will lower first before the bucket wants to dump.
To get true simultaneous action, you as the operator will have to use the control to force more fluid toward the more heavilly loaded hydraulic circuit. This can be difficult with multi lever controls, but is where a joystick truely shines.