You could add a small heat exchanger to the line going to your tankless and take a tapping off of your heating boiler. This would preheat the water going to the tankless, perhaps to a point (given the size of HXC that you choose), that the tankless wouldn’t turn on.
The system would require an additional pump on the heating side, perhaps on timer to reflect hot water demand times, although it could operate continually. Actually, if you took your tappings on the supply and return side of the boiler, you wouldn’t even need a pump.
The cost of material and install might well outweigh the benefit – but perhaps not if you’re able to buy wholesale and complete the task yourself.

Firstly you are not talking apples and apples here. The Titan is for heating potable “domestic” and your boiler is for non potable heating water. However there is a way you can save money, the initial output would be expensive. What you can do is install a domestic tank with a heat exchanger, most boiler manufacturers supply, and would need to be professionally installed, so that when you are using the boiler for heat you can be heating your domestic water as well, and when not in use you can switch the Titan heater back on.
They do not make any boilers that operate instantaneously like the titan water heater, but you can purchase one with a better efficiency rating.

I assume your boiler is for a hydronic heating system. Given this, there are a number of tankless units specifically built for hydronic heating systems. They are very common in Great Britain. The have a small tank but it is only to maintain system pressure. I am attaching a number of links to various manufactures that make tankless systems to replace boilers for hydronic heating. There is a good chance they will save you money. It depends on the cost of gas compared to the cost of electricity. In some areas like the northwest US, the cost of electricity is significantly less and it is better to have electric heat than gas heat.

tankless:@130F, boiler:180+F.in floor radiant with a commercial tankless works. More$.

Noritz has a line of gas fired tankless water heaters that can be configured to what you want. I have installed them in homes to do both domestic hot water production and heating. This would be something you should call a licensed plumbing and heating company to do.

Your tankless water heater reacts to your water demand, it doesn’t store water, so tapping an existing hot water source won’t help. If you want to save some energy dollars I would suggest attic and wall insulation, in that order.

That is what a boiler does if it is set to cold start. Some boilers maintain constant temp. You can set it to only fire when called for. I cant see a tankless water heater saving money or lasting that long with that much use.You figure how many gallons of water circulate during the coarse of a day with a boiler? It would probably burn up the tankless after a while.

what are you? “Tim, the Tool Man, Taylor”?