I have just booked my tattoo (the outline anyway)of corset laces on the backs of each leg (full leg) I have asked them to do it in parts as I currently cannot afford the whole thing (nor can I afford both full leg outline) , and I have asked if they will do the calves of both legs first, then add the top halves later. They have said that it would be "really difficult" , I beg to differ. I wouldn't be hard to keep the original design and match it back up surely? And I'm unwilling to walk around with only one full leg outline until I can afford the other! But I want a start made on it. Thanks x

Wait until you can afford to get the whole thing done.
Why do you need to get part of it done right now?

I agree with the tattooist, even with drawing it is very hard to start something and then finish it off months later. You can never get the shading and colors exactly the same.

Just be patient and start saving.

It would be difficult to start on a tattoo such as that, leave it and then return to it later, for a number of reasons. Firstly, coming back to a drawing some months after it was started will make continuity a big issue; you will almost certainly be able to tell where one starts and the other ends. As shazamataz has said, the shading and colours will not turn out exactly the same, and it will be noticeable.

Secondly, the shape of your legs could very well change between the times you have the parts done. You could lose or gain weight or muscle on your legs, which would make the tattoo look disjointed.
Also, most tattooists would be reluctant to let somebody go around with half a tattoo on their legs; it would make them look unprofessional and sloppy.

Would you like to be completing a piece of art and have to stop and start on it as and when some money drip in? There is no way that tattoo would be the best is could be, and any tattooist worth their salt won't do a half-hearted job.

Just wait until you have enough money to have the tattoos completed fully, why is there a rush to have them half done now? This is not like a back piece or sleeve, which can incorporate different designs at different times, it is a full tattoo and would not look right if it was half-completed.