when sending BTC, in instructions it clearly states that "when using 2 different btc addresses, NVOT will be send to each other according to the deposit" - but I can enter only 1 BTC address, so how does that exactly work?
It does not matter which BTC address you put into the dashboard unless you're investing with altcoins. Therefore, you could invest Bitcoin without even registering on the website. I recommend investing from the same address multiple times rather than multiple addresses as you would have to keep track of multiple private keys.
1, so if I want to invest with ethereum as well, you will send the NVOT to the bitcoin address you got the BTC from (not the one registered), AND to the BTC address registered when I send ETH to the ethereum address generated?
2, can I send ether multiple times from different addresses (to the one generated after I register my BTC address), so the NVOT received on the registered BTC address will be equal to the sum of received funds on that ethereum address?
If you invest through a different address then NVOT will be sent to that address, if you want them to send token to the registered address you must prove that it's your account, it's not complicated if you only use one address to make transactions.