i don't understand...

[kat1214young]

Try rereading the previous cases relating to BO and  FBI
But my guess is Araide-sensei and Chris Vineyard? hehehe 

[Nyarl]

http://www.dctp.ws/V34-Reader/V34-4Read/V34-4/Detective%20Conan%20Volume%2034%20File%2004%20-%20(26).jpg

in the link above, conan says that there are two more strange people, who are they?

Probably Araide and Shuuichi. Along with Jodie, they were the three "new" recurring characters Haibara saw during the Bus Hijacking when she freaked out.

Conan didn't mean Chris because he and Heiji were talking about whether Jodie could be Chris.

[Nyarl]

but how did he realized that they are strange?

Like I wrote, they probably became suspects due to the way Haibara acted on the bus.

But, if you want more...
{volume 24 - 42 spoilers}

Spoiler:

For Shuuichi, how could he not be suspicious? Shuuichi was obviously stalking/keeping an eye on them. Conan also noticed that Haibara reacted to him during another story.

For Araide, Conan himself noted that Haibara was skipping school during the medical check-ups (given by Araide). Though we're never shown Conan himself thinking about it, there's also the strange fact a doctor who was fussing so much about the way his grandmother and housekeeper would try to tough out injuries or illnesses when we first met them would send them ahead to Aomori before he could leave Tokyo.

[kat1214young]

episode 226, The Truth Behind Valentine's, mentioned two names: James Black and Akai Shuichi, so it's probably them