Can I use only one quickbook online sign-in? my company have 2 locations with different DBA and separated bank accounts. now I separately log-in, but I like to combine.

Yes, you can surely have one login credentials with multiple companies, siamwalainc.

We'll just have to update one of your companies that you want have the same User ID and Password. I'll walk you through the easy steps: 

1. Log in to company you want to change. 
2. Go to the Gear icon and select Intuit Account under the Profile section. 
3. From the Sign In & security tab, select User ID. 
4. Enter the new user ID and password. 

You can also look into this article for more details in changing your email, user ID, or password for QuickBooks Online. 

I'll be here if you need anything else. You take care always and have a great day!

Thanks for your prompt reply Catherine. Now I understand that we can use one log-in to monitor both locations. How about their data? Are they able to be merged 2 branches under one company?

Company A has 2 branches 

1st branch (with their own bank acct.) profit & lost

2nd branch (with their own bank acct.) profit & lost

Can we see the company A profit & lost  of combined data of both branches in one report?

Then it'll be easier for me to keep or sent data to my accountant.  

Yes, you can merge the two branches under a single company, @siamwalainc. I'm here to provide the steps to categorize their data in QuickBooks Online (QBO). 

I'd suggest using the location tracking feature since you have two branches in your Company A. This way, you'll be able to view their combined data within a single Profit and Loss (P&L) report. Also, this will be easy for you to send the info to your accountant.

First, let's enable the location tracking functionality in the settings. Please take note that this is only available for the Plus and Advanced versions. I'll guide you how. 

1. Go to the Settings (Gear) icon at the upper right.
2. Select Account and Settings under Your Company. 
3. Go to Advanced from the left menu. 
4. Click Categories.
5. Check the Track locations box. 
6. Choose your Location label. 
7. Click Save. 
8. Hit Done.

The screenshot below shows you the last six steps. 

Second, let's go to the Locations page to add the two branches. Here's how: 

1. Go to the Settings (Gear) icon at the upper right.
2. Select All lists under Lists. 
3. Click Locations.
4. Choose New.
5. Enter the Name of the location, such as First Branch. 
6. Check the appropriate boxes related to your first branch. 
7. Hit Save.
8. Repeat the process for the second branch. 

See the screenshot below to show you the fifth to seventh steps. For the detailed instructions, check out this article: Set Up And Use Location Tracking. 

Third, let's create transactions to keep track of their sales and accounts receivable. One of them is an invoice. Just choose the appropriate branch from the Location drop-down. View the screenshot below for your visual reference. 

Fourth, let's set up the bank accounts for the two locations. You'll need these so you can deposit the invoice payments for each branch. 

Fifth, let's run the Profit and Loss by Location report from the Reports menu. You'll be able to view the gross profit, total expenses, and net income of your two branches. I've attached a screenshot below on how it looks like in QBO. 

When you're ready, click the Email icon beside the Print button on the report. Doing so will help you easily send it to your accountant. 

I recommend customizing the report to get the specific details you need. You can also memorize it to save its current customization settings. It serves as your record and helps you quickly access it for future use. 

I'm here anytime you have other concerns. Take care always, @siamwalainc.