If I understand what you are asking.... I've never heard of running pressors at set doses and then increasing and decreasing carrier solutions for titration. I can't see how this would actually work except in the very short term (ie the short time for the carrier solution to push the inotrope in the line into central circulation) , but perhaps I am missing something in your explanation.
There are many methods of infusing inotropes and such via Swan ports and I can't say that any one method is better than the other. However, myself and most of my experienced colleges tried to use the sideport of the cordis for fluid infusions such as maintanence, bolus, blood, etc. As you are well aware, the size of this port allows for rapid infusions. I think this makes very good sense and is good practice.
As far as inotropes and pressors go, I always try to use my VIP port or infusion ports, but in reality I found that I was usually using what was available in an emergency situation and then tried to organize later when things slowed down or the patient got less complex. I always found compatibility between drugs to be a bit of an issue (using lots of vasopressin, amio, and neo) and usually found myself short of ports and using CVP and even peripheral lines with caution (ie. labelling them and making sure the person covering me did not bolus through these lines.) But then again, we used continuous cardiac output Swans in my last area of practice so the CYP port was far game.
As far as the bolusing one drug when running two together and increasing the infusion rate, you will indeed end up giving a small bolus of one drug. You can minimize this effect by "Y-ing" the medications together as close to the infusion port as possible. If your line containing both medications is short, this effect will probably be unconsequential. Remember that the volume of fluid contained in the swan lines is very small (<1cc), plus the medication in the line is already mixed.
Also, if your pressor is running very slowly a maintanence dose is probably prudent since by itself the medication might take more time than you want to travel through the swan line and reach central circulation. If you know the exact volume of your line, you could overcome this by giving a small bolus to fill the line but this makes some people nervous.