Legally how tough this method of sharing copies of the contracts?

Yes, this agreement shall be deemed concluded.
However, if You have such a maniacal idea of submenus, how do You determine what sent the copy contains the signature of an authorised person that is, not washi Pisicina - cashier of the McDuck?

For such purposes, the output of one qualified electronic signature. Purchase thereof and require the same from partners.

Without the first instance of the contract, the Customer is not entitled to pay, and the Contractor to perform the work. It is possible to use copies, but replacing the original. Base — ask the accountant.

Is it for accounting is not a document confirming the expense?

No, you need another document to confirm the execution of the works under the contract. Otherwise, this expense cannot be deducted from the tax base.

However, if You have such a maniacal idea of submenus

If counter tax audit will find discrepancies in the copies of the agreement, it may be the reason for a comprehensive check.

And to Bob it is possible at personal meeting to run. There are other ways to reduce risk should be.
Anyway for false testimony is not much softer punishment than counterfeiting, but could not prove difficult when there is correspondence, calls, translations of advance, etc.

It is in both parties can work.
If your copy of the contract is unknown whose signature and seal, the court will be deemed that YOU have forged signatures and seals, and require from a reputable "Horns and hooves" of some activity or money.

And anyway, you have no counterfeit protection, and the simple desire to come in person and sign documents.
Want protection - sign the contract with the nearest notary.

read the answers and wonder how people find it difficult to live...

and what prevents just not sign the contract if it differs from the agreed version?
agree on the condition that the parties shall exchange the signed electronic copies of the agreement, just e-mails, and for N-days originals...
well, in the extreme case of paranoia covered... make a reservation if original conditions are different from the electronic version shall prevail to the electronic version... or the fact that the Treaty shall enter into force after an exchange of originals, or something like that
and don't listen to anyone that electronic copies of documents are not accepted as evidence...

1. Yes, this agreement shall be deemed concluded.
2. If the second copy is not signed, the contract will still be deemed to be concluded. The law does not oblige to do the n-Noe number of copies of the document. Another thing is that without the original in case of dispute one of the parties can be a problem.
3. Stitching and signature on each page is not a guarantee. Stitching can be easily removed and glued again able people, and forged signatures. Once found in the court amended page of the contract where the penalty prescribed by me, was removed, and the bottom tried to get my signature.

Conclusions: if you want to deceive -- deceive. To have at least some protection, keep the evidence, which version exactly did you coordinate with the other party. A draft of the contract to which both parties had agreed. Correspondence, to discuss controversial points. It somehow helps to build a position in the court.