Yes it is possible to put an 'in line' meter in place.
You'll want an electrician to do it for you, but here is just one such provider - http://www.powermeterstore.com/index.php?s=y
Now.... here's the tricky part -
You need to check your lease / rental agreement, to see where you stand legally. If the downstairs tenant is sub-letting from you, you may have a little more leverage.
The landlady doesn't want to incur the cost of installing a separate meter, and this may be for a couple of reasons.
1) Most electric companies require you to do some sort of upgrade to the meter base, mast, etc, at the owner's expense. This cost would be doubled, because the old meter would have to be upgraded along with the new one, in most cases.
2) The wiring may not be set up in such a fashion that it can be easily split. There may be shared lights, or other circuits, for example.
Also a permit would have to be pulled, and inspections made - which could involve things other than electrical too -- so, that's most likely a whole can of worms the landlady doesn't want to open up, hehehe
A meter/power monitor like those from the link I've supplied can be used on the single appliance, 'probably' without needing a permit. If I were you, I'd call a local electrician before doing anything else, and ask about what you want done here. Obviously, you want the meter to be installed in such a way that it could not be removed, or tampered with. You'll most likely need your landlady's permission to install this type of monitor, or meter. She probably will not want to pay for the meter, or installation, either, especially if she's already got you signed into an agreement about the billing of the electrical. So, think about whether you want to offer to incur the expense yourself, to have it installed, so your neighbor can pay for the cost of running the hot tub.
You need to be aware too -- there may actually be other causes for the higher electric bill, in addition to the hot tub. Portable heaters, and higher electric rates are just a couple of things I can think of, off the top of my head. You might be embarrassed a bit, if you create a big fuss, finally get the in-line meter installed, only to discover the hot tub was only a portion of the increase.
You may want to explore some other options -- like changing the rental agreement so that you pay a flat amount, (and this could be based on the last year's history, prior to the hot tub being installed). This could be an uphill battle though, if you've already signed an agreement, saying you will split the bill.
You may need to explore other rental options to get out of a bad situation. Having another place lined up to move into, could be a persuasive argument to get the landlady to see things your way too, especially if you've been a good tenant.
Just some things to think about.
Good Luck !
