I actually just had this same problem arise at my hospital and got reprimanded for it. Two separate pumps y-ed together. However, all of our reference books, the pharmacy, and our interaction database said it was acceptable. We do not have a nursing policy related to heparin infusion either.
I've been doing a lot of research and have come to a couple of conclusions:
First, I think the general refusal among seasoned nurses to hang heparin with another medication is that they were trained in a time before IV pumps, when using the Y-site to run another medication could potential alter the rate of the heparin infusion. Running heparin without a pump would terrify me! However, with a pump, the correct dose is administered consistently, so we needn't worry about that.
I think in an ideal world, heparin would have a dedicated line. However, the world is not ideal and if a patient does not have veins that make multiple IVs a viable possibility, I think it saves the patient undo discomfort to use the y-site. Also, every invasive line increases the possibility for infection, so it may not always be best to have dedicated lines for heparin if the other medications are compatible.
We should be using evidence-based practice, not tradition, to determine our practices. I think if a hospital does not want this to happen, the policy should be clear and pharmacists as well as nurses should know about it.