Is there any stack card which provides 2*QSFP+ optical ports can be used in switch S5720-56C-EI-48S-DC?

This two model switches although have the same port, but there is difference as below:

1, forwarding performance S6720-30C-EI-24S is 720Mps, S6720S-26Q-EI-24S is 480Mps

2, S6720–30C-EI-24S supports dual power AC/DC, and extend slot, which S6720S-26Q-EI-24S not suppot.

Yes, for S5700-28P-LI-AC but it is not through the stack card, it is through the GE uplink interface.
V200R001: The last two uplink 1000Base-X optical ports can be used as stack ports.
V200R002 and later versions: The four uplink 1000Base-X optical ports can be used as stack ports.

When you configure rate limit on S6720-54C-EI-48S-AC, the following configuration is recommended:
Set the CIR, CBS, and PBS values but not the PIR value. CBS = 200 x CIR PBS = 2 x CBS = 2 x 200 x CIR = 400 x CIR The CIR value is expressed in kbit/s, and the CBS and PBS values are expressed in bytes.
Set the rate limit for outgoing packets on the interface to 10 Mbit/s. [HUAWEI] interface ethernet 0/0/1 [HUAWEI-Ethernet0/0/1] qos lr outbound cir 10240 cbs 2048000
Set the rate limit for incoming packets on the interface to 10 Mbit/s. [HUAWEI] interface ethernet 0/0/1 [HUAWEI-Ethernet0/0/1] qos lr inbound cir 10240 cbs 2048000

As for S5700, the S5700-SI supports long reach multimode (LRM) optical modules since V100R006C00SPC800. The S5700-EI supports LRM optical modules since V100R006C01.
The S5700-LI (10GE optical ports), S5710-C-LI (10GE optical ports), S5700-HI (10GE optical ports), S5710-EI (10GE optical ports), and S5720-EI (only 10GE optical ports on extended interface cards) support LRM optical modules.

If you want to know more about the solution, you can click here.
https://support.huawei.com/enterprise/en/doc/EDOC1100195305

Of course, you still need a license after the switches are stacked, and the installation method is different from that of a stand-alone machine. You can refer to this document.
https://support.huawei.com/enterprise/en/doc/EDOC1100195305

The reason is that no extended rear card is installed on the S5700.
An S5700-EI or S5700-SI can provide only two optical interfaces for a front card. If a 4-port front card is installed, an ETPB extended rear card must be installed to provide the other two ports. Otherwise, only two optical interfaces are displayed.
S5700 Switch Thunder link com

Fixed Switches Product Series Model AC and DC Power Module Mixing S3700/S5700/S6700 S3700-26C-HI Supported S5710-28C-EI Supported S5710-52C-EI Supported S5700-28C-HI Supported S5700-28C-HI-24S Supported S5700-EI (non-PoE) Supported in V200R005 S5700-SI (non-PoE) Supported in V200R005 S6700 Supported in V200R005 S5710-HI AC and DC power modules cannot be used together, but 350 W and 1150 W AC power modules can be used in one switch. S5720-HI Supported Modular Switches · When a switch does not use a PoE power module:
AC and DC power modules cannot be used together in the same type of power slots. Power modules with different power values cannot be used together either.
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· When a switch uses a PoE power module:
Version PoE Power Module System Power Module V200R005C00 and earlier versions AC AC DC power module not supported - V200R006C00 and later versions AC AC DC DC AC DC ·
AC and DC power modules cannot be installed in the slots of the same type (system or PoE) in the same switch, and the power modules of different power values cannot be used in the same switch.
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S5720 Switch Thunder link com

The Cisco switch will probably err-disable the port when you plug it in. The workaround is to add these commands:
Switch(config)#service unsupported-transciever
Switch(config)#no errdisable detect cause gbic-invalid
It'll either work or it won't!

https://www.cbo-it.de/de/produkte/blueoptics/transceiver/qsfp.html

The BlueOptics and BlueLAN brands include the following products: Optical Transceivers, Active Optical Cable and Direct Attach Twinax Cable Solutions, MPO / MTP Solutions, SAS Solutions, Stacking Cables, Twisted Pair and Fiber Optic Patch Cords, PLC Splitters, Multiplexers, Demultiplexers, as well as other data center and cloud solutions.