Is it possible that two apartments' electricity could be unknowingly linked? How to lower unknown electrical increases?
Your first step is labeling your breakers correctly.

So you need to go through the entire house and identify every outlet (either a socket or a hardwired load) and make a spreadsheet.  Plug something into every receptacle (you can usually, but not always, assume both sockets in a dual receptacle are on the same circuit).  Then move through the house, turning off everything and turning on one breaker at a time, and seeing what works with that breaker on.  

If you are good at game theory, there are more efficient ways to identify all the circuits.  

Now if you want to have some fun, you can label each breaker by a codename, like Homer, Bart, Lisa, Marge, Apu, etc. and then mark each outlet the same way.

During this testing, I'd aim for turning them all off as long as possible, only switching on the one in test.  That is to lengthen the amount of time circuits are off, to increase the chance of a neighbor complaining about his power going out. That'd be your power poacher right there. 

While you're switching breakers, keep an eye on the spin rate of your analog power meter, or the instantaneous use number on your smart meter, for any abrupt changes. 

Now, get a "Kill-a-Watt" and measure power of each pluggable device

You're looking for average watts over a long time.   This is to look for devices which may surprise you, like that "perfectly ordinary" PC that turns out to be a power hog.  

For your fixed loads, particularly your electric heaters, figure out what their energy draw is.  Almost all of them will have a "nameplate" power rating that can be found on a nameplate, or in the documentation.  If they are "Cadet style" electric baseboard heaters, typically an 8' unit is 2000W, a 6' unit 1500W, and a 4' 1000W.

You may also find "Watts" labeled as "VA" (volt-amps). There are subtle differences between W and VA, that's why there are 2 different terms.  If you can only find "Amps", look also for volts (120 or 240) and multiply amps x volts.   V * A = VA =~ W. 

Over those 2 weeks, you used 4.4kw (4400W) on average.  An "average" basic home with gas heat uses 1000W on average, so you used 3400W above that -- or almost two 2000W electric heaters burning 24x7.  Your goal is to do the detective work to find why.



Just the same, electric heat is murder. It's much more expensive than you expect it to be, because instead of getting 90% of the heat value of natural gas, you're getting like 30% with transmission losses.  

If you are in an area with a lot of nuke or hydro, talk to your power company to see if they have special rates for people with electric heat.  For instance in the south, they have lots of nuke and their peak grid stress is in the summer A/C season, so they have plans that practically give away power the rest of the time.