Principal just means a participant. In the case of asset management, a principal can mean either the party who entrusts the assets they own or the party who is entrusted with the assets.

I think here "受托管理资产所有人及公司" means the individual or the company who entrusts their assets with the trustee. If you think "the individual or the company" is too long, you can just say "owner".

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Note added at 1 day11 hrs (2016-04-25 04:04:49 GMT)
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On second thoughts, I think the meaning of 受托管理资产所有人及公司 is ambiguous. While the meaning of 受托管理资产所有人 is certain, it is not so for the latter part. If 公司 means 受托管理资产所有公司, it refers to the company who owns the entrusted asset. But if 公司 means 受托管理资产的公司, then it means the company who is entrusted with assets. In this case, I think your translation would be acceptable. However, considering that there may be other parties involved like a depository, I think it would be better to state clearly what the source refers to. I would translate it to "the owner and trustee of entrusted assets", although I know this is not concise.