can a Plaintiff use a lawyer from the same law firm as the Defendant?

Yes it is a conflict of interest. Someone will need to find a new attorney. Who ever got to the attorney's office first will get to use them, and the other will need to find a new attorney, unless their is history for both parties using them. Then both attorney's should step out, and both of you will need to find a new attorney.

It depends. Do they work for the same company, or do they merely share office space? If they work for the same firm, it is a conflict of interest. If they just happen to share an office, but don't work for the same company it is not. If you have a history with your attorney talk to them about the possibility of a conflict of interest, if you don't have history with that attorney go find a new one.

Both attorneys should have done a check for conflict of interest before agreeing to take on the case, and if they indeed work for the same firm one of them failed to do so. If you really like your attorney, it may be possible to sign a waiver that you are aware of the conflict, but still would like them to represent you. Of course that depends on the state you live in, and what the local bar association says about it.

Edit: the sooner you say something to your attorney the better. Although the lack of the other partner to do a conflict of interest check raises red flags about the entire firm in my opinion. It's possible that the office may be unaware of the conflict. Tell your attorney first, if your attorney is unwilling to get the matter settled, then yes tell the judge but it should be settled long before you enter the court room.

Yes, you have a conflict of interest. A conflict of interests exists when the representation of one client will be directly adverse to the interest of another client. In the scenario you described, both attorney's work in the same firm; hence, the creation of the conflict.

The only way this would not have a conflict of interests is if YOU gave your INFORMED CONSENT to allow this to take place. My question to you is what did YOUR attorney say about the conflict? He should have brought this to your attention and attempted to obtain your INFORMED CONSENT. Moreover, the conflict should have been brought to the attention of the court. If your attorney said nothing to you about this, then you should QUESTION if he's really acting in your best interest.

Every attorney knows (or should know) when they're in a conflict. Prior to receiving their bar license (assuming you're in the USA), they're required to certify to an understanding of legal ethics (Rules of Professional Conduct - Multistate Professional Responsibility Examination) and of the many topics covered, "Conflicts of Interest" is at the top of the list.

Each state has it's own Rules of Professional Conduct but the American Bar Association also publishes a set of Rules. I've included a link to Rule 1.7 which sets out an explanation of conflicts of interest.

If you lose in court under this scenario, you can inform the court of the conflict of interest and get the ruling set aside. Moreover, both attorney's would be subject to sanctions from the court for not obtaining your written INFORMED CONSENT prior to the litigation of the lawsuit.

I've been told it is a conflict of interest. But you had already done business through that office, right? So from what I understand her attorney should have asked your name and then checked to see if you were being represented by anyone in that office.

The plaintiff is the side that brings the case. This would be the government in criminal trials and in civil trials, the party that is suing. The defendant is the person who is facing charges in a criminal trial and the person who is being sued in a civil trial. A client is somebody who is represented by an attorney.

The fact that both lawyers work for the same law office should have no bearing on the matter as each of them are working independently.

That is interesting

Get another lawyer...quickly.