My answer is with respect to the United States. I have no idea about India's regulatory environment.
Don't take custody of the assets
You are opening yourself up to massive liabilities and problems if you deposit their money in your account.
I managed investment accounts as a private investment advisor for years (those with less than 15 clients were not required to register) until Dodd-Frank changed the rules. Thus you would have to register as an advisor, probably needing to take the series 65 exam (or qualifying some other way, e.g. getting your CFP/CFA/etc...).
I used a discount broker/dealer (Scottrade) as the custodian.
Here's how it works: Each client's account was their own account, and I had a master account that allowed me to bill their accounts and manage them.
They signed paperwork making me the advisor on their account.
I had very little accounting to handle (aside from tracking basis for taxed accounts).
If you take custody of the money, you'll have regulatory obligations.
There are always lots of stories in the financial advisor trade publications about advisors who go to jail for screwing their clients. The most common factor: they took custody of the assets.
Do you want a single account?
I understand why you want a single account - you want to ensure that each client gets the same results, right?
Does each client want the same results?
Certainly the tax situation for each is different, yes?
Perhaps one has gains and wants to take losses in one year, and the other doesn't. If their accounts are managed separately, one can take losses while the other realizes gains to offset other losses. Financial advisors offer these kinds of accounts as Separately Managed Accounts (SMAs). The advisors on these kinds of accounts are mutual funds managers, and they try to match a target portfolio, but they can do things like realize gains or losses for clients if their tax situation would prefer it.
You certainly can't let them put retirement accounts into your single account unless the IRS has you on their list of acceptable custodians.
I suggest that you familiarize yourself thoroughly with the regulatory environment that you want to operate under. Then, after examining the pros and cons, you should decide which route you want to take.
I think the most direct and feasible route is to pass the Series 65, register as an investment advisor, and find a custodian who will let you manage the assets as the advisor on the account.
Real estate is another matter, you should talk to an attorney, not some random guy on the internet (even if he has an MBA and a BS in Real Estate, which I do). This is very much a state law thing.