IV piggyback

If you're using the primary bag just to prime and flush the line then you don't actually need to set the pump for a separate secondary infusion, you can just set the pump to run at the prescribed rate but set the volume-to-be-infused to include the flush as well (if the antibiotic is 250ml then set the VTBI to 275ml or there-abouts).

Muno's method is very simple and can be used with basically any pump. Just remember to account for the amount of fluid in the line you have to push through before the med reaches the patient and that same amount of fluid will be needed after the secondary infusion is complete to flush all the med from the line. If you back prime, add that amount of fluid to the total. So if your primary set takes 20 mL to prime and your secondary set takes 15 mL to prime, you'll need to add 55 mL to the volume of the secondary infusion to get it all in. If you don't back prime, just add 40 ml.

The pumps I use are relatively smart so I just program a primary with a small VTBI that's a little over double the prime volume of the primary set. I don't back prime so that makes things easier for me. I then program the secondary as the med to be infused, and the primary rate matches the rate of the secondary rate. This way I can just look at the pump and the display will show what's infusing: primary or the med.