Short answer: Maybe. Talk to your service provider.
Long answer: Here's the FCC's answer to this question:
Yes, but there are restrictions. Each individual phone must have a unique Electronic Serial Number (ESN). The ESN is a unique number programmed into each cellular telephone at the time it is manufactured and is the means by which a cellular carrier identifies a telephone to determine whether the user of that phone is entitled to obtain service and to insure that the proper accounting is made of all activity. Most cellular phone emulators or extension services simply "clone" cellular phones, duplicating not only the telephone number but also the ESN. This activity is in violation of current Commission rules.
The Code of Federal Regulations Title 47, Section 22.915, entitled Cellular System Compatibility Specifications, generally sets forth the standards of cellular operation as reflected in the Cellular System Mobile Station-Land Station Compatibility Specification (April 1981 ed.), Appendix D to the Report and Order in CC Docket No. 79-318, 86 FCC 2d 469, 567 (1981). It is a violation of Section 22.915 of the Commission's rules for an individual or company to alter or copy the ESN of a cellular telephone so that the telephone emulates the ESN of any other cellular telephone. Moreover, it is a violation of the Commission's rules to operate a cellular telephone that contains an altered or copied ESN.
Part 22 of the Commission's rules was recently revised to add a new rule Section 22.919, to further clarify the issue of ESNs. Pursuant to subpart (c) of the referenced section, it is a violation to remove, tamper with, or change the ESN chip, its logic system, or firmware originally programmed by the manufacturer.
It currently is possible to obtain two cellular phones with the same telephone number if the cellular carrier in the market has the software in place to handle the billing and its fraud detection system has been notified not to be triggered by the use of two phones with the same phone number in suspicious circumstances.
However, there are technical reasons why it may be impossible to do this with CDMA specifically..
The phone identifies itself to the network in a registration message by including the NAM [effectively, its phone number]. From that, the network looks up its ESN. The phone knows its own ESN. The ESN never passes over the RF link in CDMA.
The long code used on the reverse link is modified by using the ESN of the phone. Since both the network and the phone know the ESN, they both modify it in the same way. As mentioned above, only one phone in existence can have any given ESN.
If a second phone tries to identify itself using the same NAM, the table lookup at the network will turn up the first phone's ESN rather than the second phone's ESN -- and the reverse link will be severely degraded (by a high "chip error rate"), probably to the point of uselessness, due to the network and phone using different ESNs to modify the long code.